Bump google-protobuf from 3.15.4 to 3.15.6 in /ruby (#136)

Bumps [google-protobuf](https://github.com/protocolbuffers/protobuf) from 3.15.4 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.4...v3.15.6)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

ruby/Gemfile.lock

@@ -15,7 +15,7 @@ GEM
     coderay (1.1.3)
     concurrent-ruby (1.1.8)
     diff-lcs (1.4.4)
-    google-protobuf (3.15.4)
+    google-protobuf (3.15.6)
     method_source (1.0.0)
     nio4r (2.5.5)
     parallel (1.20.1)